1. Field of the invention
The present invention relates to an image forming apparatus for forming images by digitalizing multivalued images using pulse width modulation.
2. Description of the prior art
Both high resolution and excellent gradation capabilities are in high demand today for both personal- and business-grade laser printers and other hard copy imaging devices. An example of a conventional image forming apparatus common today is described below with reference to FIG. 14.
FIG. 14 is a block diagram of a conventional pulse width modulation circuit used for the image forming apparatus. A D/A converter 102 converts a raster scan digital image signal 101 to an analog image signal 103. A 1/2 frequency divider 104 divides a pixel clock 119 of the digital image signal 101 in half, and outputs a screen clock 120. Pattern signal generator circuits 105, 106, and 107 output pattern signals 108, 109, and 110, respectively, based on the screen clock 120. The period of each of the pattern signals 108, 109, and 110 is twice the pixel clock 119, and waveforms of these signals are different from each other. The comparators 111, 112, and 113 compare the analog image signal 103 with each of the pattern signals 108, 109, and 110, and outputs pulse width modulated (PWM) signals 121, 122, and 123. A density gradient detection circuit 116 detects the density gradient of the digital image signal 101 in the main scanning direction, and outputs a density gradient detection signal 117. A selector 114 selects one of the PWM signals 121, 122, and 123 based on the density gradient detection signal 117, and outputs a PWM signal 115.
FIG. 15 is a timing chart of the conventional pulse width modulation circuit shown in FIG. 14. Digital image signal 101 is synchronized with rising edges of pixel clock 119. The frequency of screen clock 120 is one-half that of pixel clock 119. The voltage of analog image signal 103 which is converted from digital image signal 101 is low level when digital image signal 101 is white data, and it is high level when digital image signal 101 is black data. The pattern signal 108 is a ramp wave with a slope rising to the right. The pattern signal 109 is a triangular wave. The pattern signal 110 is a ramp wave with a slope descending to the right. PWM signals 121, 122 and 123 have duties corresponding to individual voltage levels of analog image signal 103. The density gradient detection circuit 116 shown in FIG. 14 detects the direction and steepness of the image signal density gradient, and determines which PWM signal is to be selected by the selector 114. PWM signal 115 is an output selected by the selector 114 according to a selector control input.
As thus described, this conventional pulse width modulation circuit is able to prevent a drop in the resolution of text images without jaggies appearing in the edge area with pulse width modulation of image signals containing text and other line images even though the pattern signal frequency is twice the pixel clock (see Japanese patent laid-open publication number H2-47973).
However, the screen pattern of the image thus formed is a linear screen with longitudinal lines, and, accordingly, the screen pattern is hard on the eyes when viewing.
Furthermore, the linear screen pattern affects differences in the tone upon forming halftone color images by overlaying at least cyan, magenta and yellow partial images since they are shifted in a different manner according to their relative positions on a print paper and, thereby, the rate of overlap among them is changed at individual positions.
In order to avoid these disadvantages due to the linear screen pattern, there has been proposed a method in which a screen processing is performed by giving a different screen angle for each color so that differences in the relative position on the paper do not affect differences in the tone.